The commercialization of cochlear implants, which directly stimulate the auditory nerve to provide hearing to the profoundly deaf, is somewhat recent (introduced in 1984 as an FDA-approved device). These conventional devices utilize the compound nerve action potential (CNAP) produced by the presence of an electric field in proximity of the spiral ganglion cells within the cochlea. In such conventional devices, acoustic sounds from the environment are digitized, separated into a plurality of frequency bands (called “audio-frequency channels” herein) and the loudness envelope of the signal in all of the audio-frequency channels carries the information necessary to generate electrical signals to stimulate cochlear nerves to allow the patient to perceive speech and other pertinent sounds. In electrical cochlear implants, pulsatile electric currents are modulated in amplitude to convey this information to the listener. Pulse-repetition rate and pulse width would typically be held constant, while pulse amplitude is modulated to follow relative changes in loudness. While electrical cochlear implants can be effective, they often lack the specificity to target the desired auditory nerve pathway without also activating other auditory nerve pathways as a side effect (because electrical current spreads in the body, most if not all neuromodulation devices wind up stimulating other nerves in the area besides the intended target (thus potentially causing, for example, unintended hearing sensations)). The presence of a stimulation artifact can also obfuscate signals elsewhere along the auditory nerve, which precludes stimulating and recording electrical nerve activity in the same location.
As used herein, the auditory-nerve pathway includes all of the nerves from and including the cochlea, to and including the brain stem.
The discovery that neural compound action potentials (CAPs) can be evoked by pulsed optical stimulation has led to development of cochlear implants based on optical stimulation (e.g., see U.S. Pat. No. 8,012,189 issued Sep. 6, 2011 to James S. Webb et al., titled “Vestibular Implant using Optical Stimulation Of Nerves” and U.S. Patent Application Publication US 2011/0295331 of Jonathon D. Wells et al., dated Dec. 1, 2011 and titled “Laser-Based Nerve Stimulators for, e.g., Hearing Restoration in Cochlear Prostheses and Method,” both of which are incorporated herein by reference, and both of which are assigned to Lockheed Martin Corporation, the assignee of the present invention). Optical stimulation provides more precise neural stimulation compared to electrical stimulation methods because light is directed in a single direction, and there is no stimulation artifact. However, the physiological mechanism of optical stimulation is different than that of electrical stimulation. This leads to the challenge of encoding the information for the listener in a way that optimally exploits the physiological mechanism of optical stimulation.
U.S. Patent Application Publication US 2010/0049180 of Jonathon D. Wells et al., dated Feb. 25, 2010 and titled “System and Method for Conditioning Animal Tissue using Laser Light” is incorporated herein by reference in its entirety. Wells et al. describe systems and methods for prophylactic measures aimed at improving wound repair. In some embodiments, laser-mediated preconditioning would enhance surgical wound healing that was correlated with hsp70 expression. Using a pulsed laser (λ=1850 nm, Tp=2 ms, 50 Hz (in this context, Hz means stimulation pulses per second (pps)), H=7.64 mJ/cm2) the skin of transgenic mice that contain an hsp70 promoter-driven luciferase were preconditioned 12 hours before surgical incisions were made. Laser protocols were optimized using temperature, blood flow, and hsp70-mediated bioluminescence measurements as benchmarks. Bioluminescent imaging studies in vivo indicated that an optimized laser protocol increased hsp70 expression by 15-fold. Under these conditions, healed areas from incisions that were laser-preconditioned were two times stronger than those from control wounds. Though useful for wound treatment and surgical pre-treating, chronic heating of tissue (such as the cochlea) is detrimental.
Other prior-art includes:                Qian-Jie Fu and Robert Shannon, “Effect of Stimulation Rate on Phoneme Recognition by Nucleus-22 Cochlear Implant Listeners,” J. Acoust. Soc. Am., vol. 107, pp 589-597 (2000) (hereinafter Fu et al., 2000a);        Kandel et al., Eds., “Principles of Neural Science”, McGraw-Hill Medical; 4th edition (January 2000), Ch 30-31 (hereinafter Kandel et al., 2000);        Loizou, “Speech Processing in Vocoder-Centric Cochlear Implants,” Adv. Oto-Rhino-Laryngology, vol. 64, pp 109-143, Karger, (2006) (hereinafter Loizou, 2006);        Vandali et al., “Speech Perception as a Function of Electrical Stimulation Rate: Using the Nucleus 24 Cochlear Implant System,” Ear and Hearing, vol. 21, pp 608-624, (December 2000) (hereinafter Vandali et al., 2000);        McKay, et al., “Loudness Summation for Pulsatile Electrical Stimulation of the Cochlea: Effects of Rate, Electrode Separation, Level, and Mode of Stimulation,” J. Acoust. Soc. Am., vol. 110, pp 1514-24 (September 2001);        McKay, et al., “Loudness Perception with Pulsatile Electrical Stimulation: The Effect of Interpulse Intervals,” J. Acoust. Soc. Am., vol. 104, pp 1061-74 (1998) (hereinafter McKay et al., 1998);        Littlefield et al., “Laser Stimulation of Single Auditory Nerve Fibers,” Laryngoscope, vol. 120, pp 2071-82, (2010) (hereinafter Littlefield et al., 2010);        Heinz et al., “Response Growth with Sound Level in Auditory-Nerve Fibers After Noise-Induced Hearing Loss,” J. Neurophysiology, vol. 91, pp 784-95 (2004);        Fu & Shannon, “Effects of Dynamic Range and Amplitude Mapping on Phoneme Recognition in Nucleus-22 Cochlear Implant Users,” Ear and Hearing, vol. 21, pp 227-235 (2000) (hereinafter Fu et al., 2000b);        Nelson et al., “Intensity Discrimination as a Function of Stimulus Level with Electric Stimulation,” J. Acoust. Soc. Am., vol. 100, pp 2393-2414 (October 1996) (hereinafter Nelson et al., 1996);        Omran et al., “Semitone Frequency Mapping to Improve Music Representation for Nucleus Cochlear Implants,” EURASIP Journal on Audio, Speech, and Music Processing, 2011:2 (2011) (hereinafter Omran et al., 2011); and        Fischer, “Piano Tuning,” Theodore Presser Co. (1907) (reprinted by Dover Publications, 1975) (hereinafter Fischer, 1907/1975); each of which is incorporated herein by reference.        
There is a need for an improved apparatus and a corresponding method for optical (and optionally optical combined with electrical) stimulation of nerves to restore hearing.